[PATCH 0/6] KVM: arm64: Treat 32bit ID registers as RAZ/WI on 64bit-only system

Oliver Upton
2022-08-17T21:49:34Z

For reasons unknown, the Arm architecture defines the 64-bit views of
the 32-bit ID registers as UNKNOWN [1]. This combines poorly with the
fact that KVM unconditionally exposes these registers to userspace,
which could throw a wrench in migration between 64-bit only systems.

This series reworks KVM's definition of these registers to RAZ/WI with
the goal of providing consistent register values across 64-bit machines.

Patches 1-2 clean up some of the ID register accessors, taking advantage
of the fact that the generic ones already know how to handle RAZ
registers.

Patches 3-4 wire in a new visibility bit to indicate a register ignores
writes from userspace.

Patch 5 moves all exposed 32-bit ID registers to have RAZ/WI behavior on
64-bit only systems. Note that hidden 32-bit registers continue to have
RAZ behavior and carry the additional requirement of invariance.

Lastly, patch 6 tests that userspace and guest indeed see the registers
as RAZ/WI.

Applies to 6.0-rc1 + the mismatched system fixes [2] picked up earlier
today. Tested on the fast model, both with mismatched AArch32 support
and no AArch32 support whatoever.

[1]: DDI0487H.a Table D12-2 'Instruction encodings for non-Debug System Register accesses'
[2]: https://lore.kernel.org/kvmarm/20220816192554.1455559-1-oliver.upton@linux.dev/

Oliver Upton (6):
KVM: arm64: Use visibility hook to treat ID regs as RAZ
KVM: arm64: Remove internal accessor helpers for id regs
KVM: arm64: Spin off helper for calling visibility hook
KVM: arm64: Add a visibility bit to ignore user writes
KVM: arm64: Treat 32bit ID registers as RAZ/WI on 64bit-only system
KVM: selftests: Add test for RAZ/WI AArch32 ID registers

arch/arm64/kvm/sys_regs.c | 137 +++++++++---------
arch/arm64/kvm/sys_regs.h | 24 ++-
tools/testing/selftests/kvm/.gitignore | 1 +
tools/testing/selftests/kvm/Makefile | 1 +
.../kvm/aarch64/aarch64_only_id_regs.c | 135 +++++++++++++++++
5 files changed, 222 insertions(+), 76 deletions(-)
create mode 100644 tools/testing/selftests/kvm/aarch64/aarch64_only_id_regs.c

--
2.37.1.595.g718a3a8f04-goog